Electron gun



y 1963 R. E. BENWAY 3,090,882

ELECTRON GUN Filed April 15, 1960 r 20ml ou-iml 00M200 2044 40-70;, at 4m n 0-400! 1445K 402 107 INVENTOR.

RUBERI EBENWAY wik 'wz Armm/ir 3,090,882 ELECTRON GUN Robert E. Benway, Marion, Ind., assignor to Radio Corporation of America, a corporation of Delaware Filed Apr. 13, 1960, Ser. No. 21,966 10 Claims. (Cl. 313-82) This invention relates to electron guns for cathode ray tubes of the type used, for example, in television receivers.

The type of electron gun for tubes such as described above is well known in the art and usually comprises a plurality of axially aligned electrodes mounted in fixed spaced relation usually on a plurality of glass rods. Such a gun generally includes a cathode for thermionically emitting electrons, a control electrode for modulating the flow of electrons from the cathode, a screen grid or first accelerating electrode for initially accelerating the electrons and for converging them to a crossover, and two or more additional electrodes for electrostatically focusing and further accelerating the electrons into a high velocity electron beam. The focusing action of such an electron gun is provided by two or more electrodes having different potentials applied thereto which create an electrostatic field which serves as an electron optical lens.

Electrode shapes, sizes, and spacings, as well as the voltages applied thereto, determine the characteristics of operation of the electron gun. The obtaining of one desired characteristic of an electron gun may conflict with the obtaining of another desired characteristic. Accordingly, the prior art has resorted to compromises in order to provide some degree of both characteristics in the same structure. Such compromises have, for example, been provoked by the recent desire for shorter cathode ray tubes for the purpose of permitting shallower depth television receiver cabinets.

One expedient for obtaining shorter tubes has been to provide shorter electron guns. The prior art has exercised this expedient by either shortening the electrodes or spacings thereof or by omitting one or more of the electrodes in the electrostatic focusing system of the gun. For example, it has been common to eliminate one electrode of a conventional 3-electrode focusing system. However, such elimination of an electrode results in blooming and sacrifice of depth of focus characteristics of the gun. Blooming may be generally described as the increase of beam diameter with the increase of beam current such as is produced by normal modulation thereof. Depth of focus is generally defined as the permissible range of voltage which can be applied to a focus electrode and still obtain a degree of beam focus within a predetermined acceptable range. Blooming and sacrifice of depth of focus adversely affect picture quality. Previous attempts to provide short electron guns of the 2-electrode focusing system type have resulted in rather severe blooming and shallow depth of focus characteristics.

Accordingly, it is an object of my invention to provide a novel electron gun structure which exhibits improved operating characteristics as compared with similar-type electron guns of the prior art.

Another object of my invention is to provide an electron gun of the relatively short 2-electrode focusing system variety which exhibits reduced blooming and increased depth of focus characteristics as compared with similar variety guns of the prior art.

Briefly, according to my invention an electron gun comprises, in the order named, axially aligned cathode, control grid, screen grid, focusing, and accelerating electrodes. A beam focusing action is produced primarily by an electrostatic lens created by different voltages on the focusing and accelerating electrodes. The screen grid is a cenaired States Patent 3,090,882 Patented May 21,1963

adjacent the focusing electrode, which is itself tubular and open ended. The accelerating electrode is tubular and its end adjacent the focusing electrode is partially closed. The diameter of the focusing electrode is larger than the diameter of the screen electrode and approximately equal I to the axial distance between the apertured ends of the trally apertured relatively shallow cup with its open end V accelerating and screen electrodes.

In the drawings:

FIG. 1 isa partially schematic, longitudinal section of aprior art electron gun having a conventional 3-electrode focusing system;

FIG. 2 is a partially schematic, longitudinal section of a prior art electron gun of the short 2-electrode focusing system variety described above;

FIG. 3 is an elevation ,view with parts broken away of a cathode ray tube suitable for incorporating an electron gun according to my invention;

FIG. 4 is a longitudinal section of an electron gun according to my invention;

FIG. 5 is a schematic representation of a portion of the gun of FIG. 4 together with an actual field plot of the electrostatic focusing field; and

FIG. 6 is an axial section of a modification of the screen electrode according to my invention of the gun of FIG. 4.

When compactness of the electron gun is not of primary concern, a 3-electrode focusing system gun 10, as illustrated in FIG. 1, has been widely used. Such a gun comprises a cathode sleeve 11 mounted concentrically within a control grid cup 12, a screen grid cup 14, a first anode 16, a focusing ring 18, and a second anode 20. The focusing ring 18 surrounds and overlaps adjacent ends of the two anode electrodes 16 and 20.

In one example of the operation of the gun 10, voltages of 0, 50, 300, 20 ,000, 0, and 20,000 may be applied, respectively to the electrodes 11, 12, 14, 16, 18, and 20 as illustrated. Such operation results in a converging of the electrons leaving the cathode 11 to a crossover in or near the aperture 22 of the screen grid cup 14 and then focusing and accelerating of the electrons by the 3-electrode focusing system 16, 18, and 20. The focusing field comprises a pair of electron optical'lenses, one between the focusing ring 18 and each of the anodes 16 and 20. The focusing action provided in the gun 10 exhibits an extremely great depth of focus. This might be attributable to the fact that since the focusing fields are rather much confined'within the focusing ring 18, even when the voltage on the focusing cylinder 18 is altered, the strength of the focusing lenses are not appreciably changed.

The Z-electrode focusing system gun 10 also exhibits a minimum of blooming by virtue of the high voltage anode 16 being positioned adjacent the screen grid cup 14. Due to the relative position of these two electrodes, the high voltage field from the first anode 16 extends into the aperture 22 of the screen grid cup 14, the region of the crossover of the beam. This strong positive field is believed to counteract space charge effects in the beam at the crossover and thus provides a more nearly perfect point crossover thereof.

Where compactness of an electron gun is considered a prerequisite of tube design, a 2-electrode focusing system gun 30, as illustrated in FIG. 2, has been commonly employed. The gun 30 may comprise a cathode sleeve 31 concentrically mounted within a control grid cup 32, a screen grid cup 34, a tubular focusing electrode 36 and a tubular accelerating electrode 38.

In operation of such a 2-e1ectrode focusing system gun 30, voltages of 0, 50, 400, 200, and 20,000 might be applied, respectively, to the electrodes 31, 32, 34, 36, and 38. By such operation, electrons emitted from the cathode 30 are modulated bythe control grid cup 32 and converged to a crossover by the screen grid cup 34 at or near the aperture 40 of the screen grid. The electrons emerging from the crossover are then focused and accelerated by the two electrodes 36 and 38. The principal focusing field is a single lens created between the focusing electrode 36 and the accelerating electrode 38.

The gun 30 of HG. 2 somewhat resembles the 3-electrode focusing system gun with the first anode 16 eliminated. Although resulting in a considerably shorter electron gun, the elimination of this electrode 16 also results in the sacrifice of two very desirable characteristics of gun operation, good depth of focus, and low blooming characteristics.

In the gun 30 of FIG. 2, the focusing field between electrodes 36 and 38 extends into the focusing electrode 36 toward the screen grid cup 34. As the voltage on the focusing electrode 36 is adjusted in order to obtain best operating condition of the gun, the focusing field not only changes in intensity, but also in its shape since there is nothing which confines the field. Rather, the field is permitted to extend even further into the screen grid cup 34. Accordingly, changing of the voltage on the focusing electrode 36 results in a rapid or severe changing of the focusing action of the primary focusing field. This is interpreted as a very shallow depth of focus.

Moreover, inasmuch as there is no high voltage electrode adjacent the screen grid cup 34 in the 2-electrode focusing system gun 30, there is no appreciable high voltage field which dips or extends into the crossover region of the beam. Thus, space charges are not counteracted, and a relatively poor crossover is produced. This ultimately results in rather severe blooming characteristics of the gun. Although the accelerating electrode 38 is a high votlage electrode, it is separated from the screen grid cup 34 by a low voltage electrode, i.e., the focusing electrode 36. Thus, the high voltage field from the accelerating electrode 38 cannot penetrate to the crossover region at the aperture 40 to any great extent.

I have discovered that by properly designing the electrodes of a 2-electrode focusing system gun, depth of focus can be substantially improved. This is made possible by an electrode structure wherein a relatively great change of voltage on the focusing electrode 36, although changing the intensity of the electrostatic focusing field, does not substantially change the shape of this field. Moreover, according to my invention a gun structure is provided which not only accomplishes this improvement in depth of focus but at the same time alleviates the blooming problem in prior art 2-electrode focus system guns by enabling the high voltage field from the accelerating electrode to dip well into the aperture of the screen grid cup where the crossover of the beam occurs.

FIG. 3 illustrates a suitable type of cathode ray tube for incorporating my invention. The tube 41 comprises an envelope which includes a faceplate 42, a tubular neck 44, and an interconnecting funnel 46. The neck 44 is closed with a stem structure 48 which includes a plurality of stiff lead-in conductors 50. A 2-electrode focusing system electron gun 52 according to my invention is disposed in the neck 44 and connected to the leadin conductors 50. The electron gun 52 is adapted to project a beam of electrons toward the faceplate 42. Defiection means, such as an electromagnetic yoke 53, is provided for scanning the electron beam through a raster. A luminescent screen 54, which comprises a phosphor layer 56 and a metal backing layer 58, is disposed on the internal surface of the faceplate 42. A conductive coating 60 of suitable material is disposed on the internal surface of the funnel 46- and electrically connected to the metal backing layer 58. A high voltage contact terminal 61 is provided through the funnel 46 for making electrical contact with the conductive coating 60.

FIG. 4 illustrates the Z-electrode focusing system gun 52 according to my invention. The electron gun 52 comprises a plurality of axially aligned electrodes including an indirectly heated cathode 62, a control grid electrode 64, a screen grid electrode 66, a focusing electrode 68, and an accelerating electrode 70. In the 2-electrode focusing system gun 52 the electrodes 64, 66, 68, and 70 are next adjacent to each other, i.e., there are no other electrodes which are interposed between these electrodes. For the sake of brevity and clarity, the control, screen, focusing, and accelerating electrodes 64, 66, 68, and 70, respectively, will hereinafter be referred to simply as the G1, G2, G3, and G4 electrodes, respectively.

The various electrodes of the electron gun 52 are mounted according to well-known techniques. The cathode 62, which comprises a tubular member closed at one end, is supported within the cup-shaped G1 by a centrally apertured ceramic disk 72. The G1, G2, G3, and G4 are supported in spaced relation on a pair of insulator rods 74 into which U-shaped metal studs 76, fixed to the various electrodes, are bonded. Two of the stiff lead-in conductors 50 are directly connected to the G1 studs 76 and thus support the cathode end of the gun 52. The other end of the gun 52 is supported within the neck 44 by a plurality of spring contact members 78 which are fixed to a top aperture plate portion 80 of the G4.

Operating potentials are applied to all of the electron gun electrodes except the G4 through the lead-ins 50. G4 is energized through the high voltage terminal 61, the conductive coating 60 (FIG. 1), and the contact springs 78. However, for purposes of schematic illustration, schematic lead lines are shown in FIG. 4 to illustrate one set of suitable operating voltages for the electron gun 52. As therein illustrated, the following D.C. voltages may be applied: Cathode 62, 40-70 volts; G1, 0 volts; G2, 400-500 volts; G3, 0-400 volts; and G4, 14,00020,000 volts.

In the operation of the electron gun '52 electrons are emitted from the cathode 62 and accelerated through aligned apertures 82 and 84 in the G1 and G2, respectively. The electrons converge to a crossover in the vicinity of the G2 aperture 84 and then diverge therefrom. The divergent electrons are then reconverged and focused upon the luminescent screen 56 by a strong electrostatic focusing field established by the potentials on the G3 and G4 electrodes.

In accordance with my invention and as shown in detail in FIG. 5, the above-stated objects are attained by a novel arrangement and structure of the G2, G3, and G4 electrodes. The G2 comprises a relatively shallow apertured cup open toward the G3. The G3 comprises a tubular member open at both ends. The G4 is likewise a tubular member but includes a transverse end portion 86 adjacent the G3 and has a central aperture 88 therein providing a relatively small aperture for the electron beam as it enters the G4. The transverse end portion 86 extends slightly into the G3. The diameter of the G2 is larger than the G4 aperture 88; the diameter of the G3 is even still larger than the diameter of the G2; and the axial distance between the ends of the G2 and G4 closest to the cathode, i.e., their apertured ends, is approximately equal to or less than the diameter of the G3. The shaping and arrangement of the G2, G3, and G4 electrodes shown in FIG. 5 resultsin the creation of an electrostatic focusing field schematically shown and the shape of which is not as severely altered in response to a substantial adjustment change of the voltage on the G3 as is the case with similar type prior art guns. Thus, depth of focus is substantially increased as compared to similar type prior art guns such as the gun 30 of FIG. 2, Moreover, such novel electrode shaping and arrangement also results in the creation of a high voltage focusing field which extends into the crossover region of the beam (in the region of the G2 aperture 84) to provide a decrease of blooming as compared to similar type prior art guns such as the gun 30 of FIG. 2.

In FIG. 5 relative size and spacing dimensions of the various electrodes are designated as follows:

Diameter of G3 D1 Diameter of G2 D2 Diameter of G2 aperture 84 D3 Diameter of G4 aperture 88 D4 Axial spacing between apertured ends of G2 and G4 electrodes L Axial spacing between the G2 and G4 L1 The axial depth of G2 L2 As hereinbefore stated, improvement in blooming is obtained by providing a high voltage field in the vicinity of the G2 aperture 84. According to my invention such a field is produced in the region of G2 aperture 84 by:

With respect to feature 1, an established approximation is that an electrostatic field is diminished to about 1'0 percent its strength at a distance equal to the diameter of an aperture through which it extends, e.g., a condition of L=D1. Thus, according to my invention a field of approximately 14004000 volts percent of G4 potential) extends into the region of the G2 aperture at the crossover of the beam. This contributes greatly to alleviating blooming problems.

Also, as hereinbefore stated, improvement in depth of focus is obtained by creation of a focusing field which does not change its overall shape too severely with a substantial adjustment change of the voltage on G3. According to my invention such a field is produced by making D1 larger than D2 and L2 of a size so that the rim 90 of the G2 extends almost to an equipotential surface 92, which when undistorted from a smooth-curved cupped contour extends a distance substantially equal to L. By virtue of this relationship of G2 and G3 as illustrated in FIG. 5, the presence of the G2 does not distort the equipotential surface 92. Thus, even when the focusing field is intensified by a change of the voltage on G3, the shape of the field does not materially change. The equipotential surfaces merely crowd closer together.

The general relationship of electrode shapes and spacings according to my invention which make possible this nonchanging field shape in response to an adjustment of G3 voltage may be described in several ways. The improved nonchanging field feature can be generally obtained by an electrode arrangement in which the G2 cup has a depth approximately one-fourth its diameter and in which G3 is greater in diameter than the G2 and in which the distance between the ends of the G2 and G4 closest to the cathode is approximately equal to the diameter of G3. This relationship may be somewhat more specifically defined by a dimensional ratio wherein the ratio of L2:D2:D1:L is approximately equal to 1:4:5 :5.

The functional requirement according to the practice of my invention may likewise be defined in various ways. For example, the above-described relationship of electrode sizes, shapes, and spacings are such that the equipotential surfaces which extend into the G2 cup are substantially smooth-curved, cup-shaped imaginary surfaces, i.e., they possess no sharp changes in radius of curvature. In this respect the equipotential surfaces 92 extending into the G2 cup possess generally the same type contour as the equipotential surfaces 94 extending halfway through the G3. As illustrated by the equipotential surface 92, a smooth-curve, cup-like imaginary surface is defined by the G2 aperture 84, the G2 rim 90, and the internal cylindrical surface portion 96 of the G3 radially opposite the apertured end of the G4. These points thus serve to maintain the shape of thefocusing field constant when its intensity is changed.

In contrast to the smooth-curve, cup-like uniform field established by the gun 52 according to my invention, the field 97 illustrated in FIG. 2 is typical of operation of prior art guns of the same general class. As shown in FIG. 2, the equipotential surfaces which extend farthest into the focusing electrode 36 are distorted into a flattened shape bulged at their side adjacent the screen electrode 40. Inasmuch as the distorted equipotential surfaces are those of farthest extent, such distortion appears, or becomes most pronounced, when the field is made more intense. This means that as the field 97 is increased in intensity its shape is changed. Such changing shape is one factor which contributes to the small depth of focus characteristic of prior art guns of this general class.

As is well known, an electrostatic field established between two circular electrodes comprises a convergent portion and a divergent portion. Because the electrons of a beam passing therethrough are either being accelerated or decelerated, they are always influenced to a greater extent by the convergent portion. Thus, the field always exerts an overall convergent action. When the focus voltage is changed, both the convergence and divergence forces are changed; they are both increased or both decreased. However, because of the inherent dominating characteristic of the convergent portion of the field, the convergence forces always experience a greater change than do the divergence forces. Hence, overall convergence is either increased or decreased depending upon whether the focus voltage is decreased or increased. According to my invention, the overall net change in convergence is reduced by a gun structure in which the change of convergence is made less pronounced and the change of divergence morepronounced than is the case for prior art guns of the same general class.

In the case of an accelerating field such as is established between the G3 and G4 electrodes of the .gun 52, the convergent portion is the first portion, i.e., the portion 98 extending toward the G2; the divergent portion is that portion 100 which extends in the opposite directron into theG4. Accordingly, it is the convergent portion 98, or the portion generally contained by the G2 and G3, whose shape is maintained relatively constant in accordance with the teaching of my invention. This maintenance of shape is primarily effective to prevent distortions in the convergent field portion 98 and thereby reduce or avoid aberrations and the like.

Although the strengthof the convergent action of the field portion 98 is still affected by a change of the G3 voltage, the. effect is not as drastic as would be the case f the field portion 98 were to change its shape. This is true, since the strength of a field varies according to both the sharpness of curvature and the intensity or closeness of spacing of the equipotential surfaces. There-v fore, slnce the curvature of equipotential surfaces of the convergent field portion 98 are maintained substantially constant according to my invention, the strength of the convergent field portion 98 is increased only due to the increase of the intensity of the field.

On the other hand this is not so with the divergent field portion 100. When the G3 voltage is changed, the d-ivergent field portion 100 changes both in shape and in intensity (spacing between equipotential surfaces). Thus a change in G3 voltage affects the strength of the diver: gent field portion in two ways while it affects the strength or the convergent field portion in only. one way. In addit on to this, the G4 aperture 88, according to my inventron, is made relatively small (i.e., compared to the diam eter of the G3), thus making the radius of curvature of the divergent field equipotential surfaces relatively small, thus producing a relatively strong divergence action as an electron passes from one equipotential surface to another. Accordingly, divergence is affected by a greater number of factors than is convergence. However, since convergence in any lens inherently dominates divergence, the net result is that somewhat of a balance is obtained wherein the overall net convergence action of the entire electrostatic focusing field does not change at as fast a rate as it otherwise would, and in fact does, in prior art guns of the same general class.

This means that a greater change of the voltage on G3 can be tolerated without exceeding the permissible range of focusing strength of the focusing field. This, in turn, means that thedepth of focus has been increased.

The relative size of the diameter of the G4 aperture 88 is not critical. Generally speaking, the smallness of D4 simply determines the degree of the above-described balance of change in strength of the convergent and divergent field portions 98 and 100. I have found that a ratio of D4:D1 equal to approximately 1:3 gives quite satisfactory results.

According to a preferred embodiment of my invention, electron gun 52 comprises an electrode arrangement of the following dimensions in inches:

D1=0.500 L=0.460 D2= O.375 L1=0.358 D3=0.025 L2=0.102 D4=0.175

This results in a preferred ratio of L2:D4:D2:D1:L of l:l.72:3.68:4.90:4.50. This, thus, can generally be approximated as a ratio of 122:4:5z5.

This invention has been described by defining certain electrode relationships; specific ratios of electrode size and spacing have been given to define a preferred embodiment. However, the broad concept of the invention can be practiced with considerable variance from these stated preferred ratios. For example, the principle of my invention can be practiced in a gun structure whose ratio of D1:D2 is any practical amount greater than unity even though it is less than the preferred 500/375 or 1.33. Likewise, my invention may be practiced to degrees less than preferred when the ratio of D1:L is made greater than that shown by prior art 2-electrode focusing system guns even though the ratio is somewhat more or less than the preferred 500/460 or 1.09. Generally speaking, the invention may be practiced by any substantial change of electrode size and spacing relationships from that of prior art 2-electrode focusing system guns toward the stated preferred relationships according to my invention so as to obtain material improvement of depth of focus and decrease of blooming.

FIG. 6 illustrates a modification of the G2 electrode according to my invention which is preferred from the standpoint of performance. In the modification of FIG. 6, a G2 cup 101 includes a concave member 102 which forms the exposed interior surface thereof. The concave member 102 may be provided either as a separate, centrally apertured, cup-shaped insert element, as shown, or as an integral part of the G2 cup 101. Although the G2 cup 101 is shown to include an open space 104 between the insert 102 and the basic cup housing 106, this may in fact be filled such as by the provision of a solid one piece G2. In any of the alternate forms herein described, the modified G2 electrode 101 is characterized by an interior surface 107 which is opened toward the G3 and which comprises a continuous, smooth-curved, cup-shaped surface. This is in contrast to the G2 electrode 66 which is characterized by a cylindrical wall portion and a transverse end wall substantially perpendicular to the cylindrical section.

The modified G2 electrode 101 has proved to give somewhat better performance as compared to the G2 electrode 66. This results from the fact that the continuous curved concave surface 107 more nearly corresponds in shape to the convergent field portion 98 than does the interior of the G2 electrode 66. Thus, as the voltage on the G3 is changed for focus adjustment and 8 the intensity of the convergent field portion 98 is increased or decreased, even less distortion of shape results by use of the G2 electrode 101 than by the G2 electrode 66.

While the G2 electrode 101 is preferred from the standpoint of performance, the simpler G2 66 may still be preferred from the standpoint of economy. Depending upon the particular electron gun into which the electrode is incorporated and the compelitive elements involved in the manufacture thereof, it may be preferred to avoid the added expense of providing the insert element 102. Thus, whereas the concave G2 101 may be preferred in some instances, the simpler straight walled G2 cup 66 may be preferred in others.

I claim:

1. An electron gun comprising axially aligned cathode, contol, screen, focusing, and accelerating electrodes next adjacent to each other in the order named; said screen electrode comprising a centrally apertured cup open toward said focusing electrode; said focusing electrode comprising a tubular member open at both ends; said accelerating electrode comprising a tubular member having a transverse end section adjacent said focusing electrode, said end section having an aperture therein; the diameter of said focusing electrode being larger than the diameter of said screen electrode cup and being at least approximately equal to the axial distance between the ends of said screen and accelerating electrodes closest to said cathode.

2. An electron gun comprising axially aligned cathode, contol, screen, focusing, and accelerating electrodes next adjacent to each other in the order named; said screen electrode comprising a centrally apertured cup open toward said focusing electrode; said focusing electrode com.- prising a tubular member open at both ends; said accelerating electrode comprising a tubular member having a transverse end section adjacent said focusing electrode, said end section having an aperture therein; the diameter of said focusing electrode being larger than the diameter of said screen electrode cup and being at least approximately equal to the axial distance between the apertured ends of said screen and accelerating electrodes, the diameter of said screen electrode cup being larger than the diameter of said accelerating electrode aperture.

3. An electron. gun comprising axially aligned cathode, control, screen, focusing, and accelerating electrodes next adjacent to each other in the order named; said screen electrode comprising a centrally apertured cup open toward said focusing electrode; said focusing electrode comprising a tubular member open at both ends; said accelerating electrode comprising a tubular member having a transverse end section adjacent said focusing electrode with an aperture therein; the diameter of said focusing electrode being larger than the diameter of said screen electrode cup and being at least approximately equal to the axial distance between the apertured ends of said screen and accelerating electrodes, and being at least approximately four times the diameter of said accelerating electrode aperture.

4. An electron gun comprising axially aligned cathode, control, screen, focusing, and accelerating electrodes in the order named; said screen electrode comprising a centrally apertured cup open toward said focusing electrode; said focusing electrode comprising a tubular member open at both ends; said accelerating electrode comprising a tubular member having a transverse end section adjacent said focusing electrode with an aperture therein; the ratio of the axial length of said screen electrode cup, to the diameter of said screen electrode cup, to the diameter of said focusing electrode, to the axial distance between the apertured ends of said screen and accelerating electrodes, being approximately equal to 1:4:5:5.

5. An electron gun comprising a plurality of circularly symmetric electrodes arrayed in axial alignment next adjacent to each other in the order named and including,

a cathode, a control grid cup enclosing said cathode and having an 'apertured closed end facing in the direction of beam travel, a centrally apertured screen grid cup disposed with its open end facing in the direction of beam travel, an open ended tubular focusing electrode, and a tubular accelerating electrode having its end adjacent said focusing electrode partially closed by a centrally apertured member extending into said focusing electrode, said focusing electrode having a diameter larger by a predetermined amount than said screen grid cup, and said focusing electrode and said screen grid cup having predetermined axial lengths and being spaced by i2. predetermined distance, said predetermined dimensions being such that the edge of the aperture of the said screen grid cup, the rim of said screen grid cup, and the end portion of the internal cylindrical surface of said focusing electrode radially opposite the accelerating electrode aperture lie in a substantially smooth-curved, cup-shaped imaginary surface.

6. An electron gun comprising next adjacent to each other in coaxial alignment in the order named a cathode, an axially apertured control grid cup open toward said cathode, an axially apertured screen grid cup disposed with its open end facing away from said cathode, a focusing cylinder electrode, and an accelerating cylinder electrode having its end adjacent said focusing electrode partially closed, said screen grid and focusing electrode being adapted to be operated at potentials in the same order of magnitude and said accelerating cylinder being adapted to be operated at a potential many times higher, the diameter of the screen electrode cup being smaller than the diameter of the focusing electrode, and the axial length of the screen electrode and the axial relationships of the screen and focusing electrodes being such that the screen grid aperture, screen grid open end, and the end portion of the focusing electrode inside cylindrical surface radially opposite said partially closed end of said accelerating electrode when energized cause the equipotential surfaces extending into said screen grid cup to be smoothcurved, cup like imaginary surfaces.

7. The electron gun according to claim and wherein the internal surface of said screen grid cup comprises a continuous smooth curved, cup-shaped surface.

8. The electron gun according to claim 6 and wherein the internal surface of said screen grid cup comprises a continuous smooth-curved, cup-shaped surface.

9. An electron gun' for a cathode ray tube comprising taxially aligned cathode, control, screen, focusing and accelerating electrodes in the order named, said screen electrode comprising a centrally apertured cup open toward said focusing electrode, said focusing electrode comprising a tubular member, said accelerating electrode comprising a tubular member having a transverse end section adjacent said focusing electrode, said end section having an aperture therein and wherein:

D1=the diameter of the focusing electrode D2=the diameter of the screen electrode L=the axial spacing between the apertured ends of the screen electrode and the accelerating electrode L2=the axial depth of the screen electrode and 'wherein D1 is greater than either L or D2 and L2 is approximately of D2.

10. An electron gun for a cathode ray tube comprising axially aligned and coaxial cathode, control, screen, focusing and accelerating electrodes in the order named, said screen electrode comprising a centrally apertured cup-shaped member open toward said focusing electrode, said focusing electrode comprising a tubular member open at both ends, said accelerating electrode comprising a tubular member having a transverse end section adjacent said focusing electrode, said end section having an aperture therein, said focusing electrode being of larger diameter than said screen and accelerating electrodes, said screen, focusing, and accelerating electrodes being so formed and dimensioned that the aperture in said screen electrode, the lip of said cup-shaped screen electrode, and the inner wall of said focusing electrode adjacent said accelerating electrode aperture, lie in an imaginary surface, the axial longitudinal section of which is generally semi-elliptical.

References Cited in the file of this patent UNITED STATES PATENTS 2,922,072 Collins Jan. 19, 1960 

1. AN ELECTRON GUN COMPRISING AXIALLY ALIGNED CATHODE, CONTROL, SCREEN, FOCUSING, AND ACCELERATING ELECTRODES NEXT ADJACENT TO EACH IN THE ORDER NAMED; SAID SCREEN ELECTRODE COMPRISING A CENTRALLY APERTURED CUP OPEN TOWARD SAID FOCUSING ELECTRODE; SAID FOCUSING ELECTRODE COMPRISING A TUBULAR MEMBER OPEN AT BOTH ENDS; SAID ACCELERATING ELECTRODE COMPRISING A TUBULAR MEMBER HAVING A TRANSVERSE END SECTION ADJACENT SAID FOCUSING ELECTRODE, SAID END SECTION HAVING AN APERTURE THEREIN; THE DIAMETER OF SAID FOCUSING ELECTRODE BEING LARGER THAN THE DIAMETER OF SAID SCREEN ELECTRODE CUP AND BEING AT LEAST APPROXIMATELY EQUAL TO THE AXIAL DISTANCE BETWEEN THE ENDS OF SAID SCREEN AND ACCELERATION ELECTRODES CLOSEST TO SAID CATHODE. 